Single neurons in the mammalian central nervous sytem (CNS) contain more than one neurotransmitter. Since environmental signals alter neurotransmitter gene expression, the existence of multiple neurotransmitter systems within single neurons increases the potential complexity of neuronal responses. We, therefore, plan to study the coordinate regulation of three neurotransmitters known to be co-localized in neurons of the ventral medualla of mouse and rat brain: the "classical" monoamine transmitter serotonin (5 - HT), the peptide transmitters substance P (SP) and thyrotropin-releasing hormone (TRH). cDNA probes for the SP and TRH preprohormones are available. The application of molecular biological approaches to the problem of neurotransmitter expression will allow us to examine these issues at the level of gene expression. Armed with the tools of both neuropharmacology and molecular biology, then, we plan 1) to determine whether pharmacological alterations in serotonin levels affect the expression of co-localized peptide neurotransmitters, and 2) determine whether the observed changes in peptide expression are the result of altered serotonergic neurotransmission. Studies in vivo will be extended by studies performed in organotypic tissue culture where the microenvironment can be more carefully controlled.